Digital or solid state lighting technologies, i.e., illumination based on semiconductor light sources, such as light-emitting diodes (LEDs), offer a viable alternative to traditional fluorescent, high-intensity discharge (HID), and incandescent lamps. Functional advantages and benefits of LEDs include high energy conversion and optical efficiency, durability, lower operating costs, and many others. Recent advances in LED technology have provided efficient and robust full-spectrum lighting sources that enable a variety of lighting effects in many applications.
Some of the fixtures embodying these sources feature a lighting module, including one or more LEDs capable of producing white light and/or different colors of light, e.g., red, green and blue, as well as a controller or processor for independently controlling the output of the LEDs in order to generate a variety of colors and color-changing lighting effects, for example, as discussed in detail in U.S. Pat. Nos. 6,016,038 and 6,211,626. LED technology includes line voltage powered luminaires, such as the ESSENTIALWHITE series, available from Philips Color Kinetics. Such luminaires may be dimmable using trailing edge dimmer technology, such as electric low voltage (ELV) type dimmers for 120 VAC line voltages (or input mains voltages).
Many lighting applications make use of dimmers. Conventional dimmers work well with incandescent (bulb and halogen) lamps. However, problems occur with other types of electronic lamps, including compact fluorescent lamp (CR), low voltage halogen lamps using electronic transformers and solid state lighting (SSL) lamps, such as LEDs and OLEDs. Low voltage halogen lamps using electronic transformers, in particular, may be dimmed using special dimmers, such as ELV type dimmers or resistive-capacitive (RC) dimmers, which work adequately with loads that have a power factor correction (PFC) circuit at the input.
Conventional dimmers typically chop a portion of each waveform of the input mains voltage signal and pass the remainder of the waveform to the lighting fixture. A leading edge or forward-phase dimmer chops the leading edge of the voltage signal waveform. A trailing edge or reverse-phase dimmer chops the trailing edges of the voltage signal waveforms. Electronic loads, such as LED drivers, typically operate better with trailing edge dimmers.
Unlike incandescent and other resistive lighting devices which respond naturally without error to a chopped sine wave produced by a phase-cutting dimmer, LEDs and other solid state lighting loads may incur a number of problems when placed on such phase chopping dimmers, such as low end drop out, triac misfiring, minimum load issues, high end flicker, and large steps in light output.
In addition, dimming ranges (i.e., the range between minimum and maximum phase angles of a dimmer) may differ from dimmer to dimmer, depending on various factors, such as the model and/or type of dimmer. For example, among conventional dimmers, the root mean square (RMS) voltage output by the dimmer and seen at an input of a power converter may vary from about 45 percent to about 20 percent of the full unchopped mains at the minimum dimmer settings (corresponding to minimum dimmer phase angles and lowest levels of light output), and from about 75 percent to about 95 percent of the full unchopped mains at the maximum dimmer settings (corresponding to maximum dimmer phase angles and highest levels of light output). These differences result in various dimming levels and dimming ranges, depending on the dimmer.
FIGS. 1A and 1B depict representative chopped waveforms of a rectified input mains voltage received by a power converter from different types of dimmers (Dimmer A and Dimmer B), respectively set at their minimum dimmer settings. As shown in FIGS. 1A and 1B, the phase angle of Dimmer A at its minimum dimmer setting is larger than the phase angle of Dimmer B at its minimum dimmer setting. For example, Dimmer A may be a 6615-POW dimmer and Dimmer B may be a DVELV-303P dimmer, both available from Leviton Manufacturing Co., in which case Dimmer A will dim down only to about 17 percent, while Dimmer B will dim down to about 6 percent. The phase angle of each dimmer corresponds to an “on-time,” which is the amount of time each chopped signal waveform of the rectified input mains voltage is non-zero. The on-time may be determined, for example, by the amount of time the electronic switch of the respective dimmer is “on” (i.e., enabling current to flow to power converter). Referring to FIGS. 1A and 1B, the on-time Tona of Dimmer A is greater than on-time Tonb of Dimmer B.
Accordingly, Dimmer A provides a larger RMS voltage to the input to the power converter than Dimmer B, resulting in more light output from the solid state lighting load when Dimmer A is set at its minimum dimmer setting than when Dimmer B is set at its minimum dimmer setting. Because of the non-linear nature of the human eye's response to light intensity, the difference between the two lowest dimmer setting intensities will be dramatic. A similar situation exists with respect to the maximum dimmer settings of Dimmer A and Dimmer B.